26 yr old hvac

I have a very old HVAC unit and i am pretty sure the AC part is refusing to start this year. As oppposed to calling for a repair, I would think a replacement would be worth it at this point due to the leaps made in efficiency now. I found out that natural gas is available in my neighborhood though my house is not tapped in at this point. At the moment, I have a split unit with the indoor part located in the space over the garage which I hope to turn into a small mother-in-law suite next year. I've heard alot of talk about energy saving units, but what I want to know is What is the best priced option that will give me the lowest monthly bills? Gas, Electric, dual source, split unit, all outdoor package unit? I'm thinking of leaving the HVAC unit cooling and heating the main house and having a seperate small wll unit for the mother in law suite so that it can be heated and cooled only when in use.
Thank you for any help

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26 yr old hvac

going gas or elec will depend on utility rates in your area.Most units are at least 13-14 seer . AQs the seer value goes up so does the price. Gas package units are only 80% on heat. A mini split may be a good choice for the inlaw suite. Again it depends on utility rates in your area. I would call several HVAC companies for quoates & references

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26 yr old hvac

Find out if the utility is going to charge you $$ and how much per foot to run the line to your house. Amortize that cost over the amount of time you intend to stay there along with the cost of the equipment. If you plan to be there 15-20 yrs then higher efficiency units are worthwhile. Get a proper heat load/loss calculation done on the house to size the equipment properly. A minisplit for the motherinlaw is the best bet.